The formation of myelin sheath in the CNS by oligodendrocytes (OL) is a complex developmental process with numerous intermediate steps. Programmed myelin gene expression is regulated by both genetic factors intrinsic to these cells,and by environmental influences encountered as the cells proliferate and migrate from their sites of embryological origin to their neurons. During this period OL progenitor differentiation proceeds along a specific lineage pathway in a highly regulated sequence of events, resulting in the formation of myelinated neuronal axons. This project aims to increase our understanding of how extrinsic factors regulate OL differentiation and myelin formation by identifying and characterizing environmental factors that regulate myelinogenesis,and by analyzing the cellular and molecular mechanisms by which they operate. The project focuses on events occuring during the early stages of myelinogenesis,during which progenitor cells become committed specifically to the lineage,and differentiate into identifiable OL. A critical step in this developmental lineage,characterized experimentally by the expression of the O4 antigen but the absence of galactocerebroside (i.e., O4+GalC- ),will be examined. During this stage the progenitor cells are triggered to initiate the cascade of terminal differentiation,heralded by the expression of galactocerebroside,and followed sequentially by a complement of myelin-specific structural components. We have recently developed a procedure for the immuno-isolation of O4+GalC- cells. Two subpopulations exist within the isolated O4+GalC- progenitor population, a major (80%) subpopulation of "proligodendrocytes" that rapidly and synchronously differentiate into authentic OL in culture,and a minor (20%) subpopulation of "procrastocytes" that remain GalC-. These two coexisting populations will be further characterized with the regard to the environmental regulation of their survival,proliferation,migration,and differentiation,by both previously identified "broad spectrum" growth factors and by novel activities that we have demonstrated in cell- conditioned media. These studies will emphasize the analysis of the cellular mechanisms involved,and the purification and characterization of previously unidentified factors. We have shown that the differentiation of OL progenitors in dissoclated cultures of rat brain can be either stimulated or reversibly inhibited during this critical developmental stage by OL-specific monoclonal antibodies,in particular anti-galactolipids. We have hypothesized that the target antigens act as receptors and ligands in the response of OL to their environment. An analysis will be carried out to determined (a) the cellular and biochemical mechanisms by which these antibody pertubations proceed,(b) the identification of the molecular identities of the cell surface complexes of which these antigens are a part,and (c) the target binding sites on OL themselves and on other cells with which these oligodendrocyte surface antigens interact. These data will contribute to our understanding of a critical step of OL differentiation,thereby providing useful clues to processes critical to myelin formation,maintenance,and remyelination in both the normal and pathological state. In particular,these data are expected to bear on the stimulation of remyelination in disease states such as Multiple Sclerosis by providing information of the information of the environmental requirements for the successful transplantation of OL progenitors and the activation of dormant progenitors that appear to exist in the adult CNS.